The invention relates to a voltage-controlled oscillator provided with a frequency-determining element and an oscillator loop having a phase-rotating (or phase-shifting) network, in which the value of one or more elements depends on a control signal which is fed to a control input and with which the phase rotation and thereby the oscillator frequency can be changed. Such a voltage-controlled oscillator is known in practice.
In such an oscillator the frequency is controllable by means of a control voltage or control current, and these oscillators are usually known as voltage-controlled oscillators or VCOs. Such an oscillator is in principle made up of an amplifier which is provided with the frequency-determining element, such as a crystal or LC circuit, and which preferably has a transfer function as frequency-independent as possible, to which amplifier a phase-rotating network is connected in cascade, and one or more loops which ensure the correct oscillating, action. Such a phase-rotating network has a frequency-dependent transfer function and can be, for example, an all-pass or bandpass network, the variable elements of which are formed by, for example, the resistance of a field effect element, the transconductance of an active component or the capacitance of a so-called capacitance diode.
The controllability of the oscillator frequency is, for example, required for frequency modulation and frequency demodulation applications and in oscillators whose frequency must remain very stable. The natural frequency of the frequency-determining element, for example a crystal, depends on the temperature. Adjustment of the above-mentioned phase rotation in the network of the known oscillator with a control signal derived from a temperature sensor in such a way that there is compensation for the frequency variations caused by the temperature changes, produces a temperature-compensated oscillator or TCO.
The values of the adjustable elements in the phase-rotating network of these VCOs and TCOs are generally not known accurately, particularly if monolithic technology is employed. This means that accurately establishing the idling frequency of the oscillator in the absence of a control signal and establishing the sensitivity (the relationship between control signal and frequency change) is a great problem.
In order to cause the oscillator to oscillate at the idling frequency of the frequency-determining element, it is necessary to set the phase rotation in the phase-rotating network accurately to a value of 0 or .+-..pi.. It is thereby assumed that the amplifier shows no (excess) phase shift other than due to an inversion or non-inversion of the signal. The excess phase shift of the amplifier is assumed to be included in the phase shift of the phase-rotating network.
These circuits therefore normally need one or more adjustments of the idling frequency and sensitivity. Even if through such an adjustment the frequency is set exactly to its idling value at a particular temperature and the sensitivity is set at the desired value, due to the fact that the elements of the phase-rotating network have a certain temperature dependence, frequency variations, which are not fully determined by the frequency-determining element, may occur with temperature, and the sensitivity may vary with temperature.
This frequency drift, which is not established by the frequency-determining element and is therefore difficult to forecast, and the fact, that the sensitivity is not accurately known, make it difficult to determine the correct voltage course for compensating for the frequency drift or for a linear and accurate frequency modulation at the control input of the oscillator. This makes one or more further adjustments necessary in order to obtain a satisfactory temperature compensation or an accurate and linear frequency modulation. Such adjustments are time-consuming and expensive. The phase-rotating network which is provided to permit control of the oscillator frequency also causes the phase noise behavior of the oscillator to deteriorate as a result of the noise generated by the network.